BSTJ 48: 10. December 1969: Physical and Transmission Characteristics of Customer Loop Plant. (Gresh, Philip A.)

Physical and Transmission Characteristics
of Customer Loop Plant

By PHILIP A. GRESH

(Manuscript received June 17, 1969)

This report covers the principal physical and transmission character-
istics of the Bell System customer loop plant. Items covered include a
statistical characterization of physical composition, measured and cal-
culated transmission characteristics, and measured noise and crosstalk
performance. A survey conducted in 1964 provided the data base for this
report and comparisons of data obtained from a similar survey in 1960
illustrate that, in many respects the composition of loop plant changes only
slowly with time. Consequently, the 1964 survey results are believed to be
representative of today's plant.

The types of analyses presented in this paper are of increasing interest
to certain Bell System customers because of the increasing number and types
of services provided over local telephone facilities.

I. INTRODUCTION

This report covers the principal results of the 1964 Bell System cus-
tomer loop survey. This survey provides a statistical characterization
of physical composition, measured and calculated transmission char-
acteristics, and measured noise and crosstalk performance of customer
loop plant. Comparisons of data obtained from the 1964 survey and
a similar survey made in 1960 are also presented.

Several of the principal transmission characteristics of Bell System
customer loop plant as denned by the 1960 loop survey were published
in 1962 by R. G. Hinderliter. 1 Additional published data on the trans-
mission characteristics of Bell System toll connections is available in a
BSTJ article by I. Nasell. 2

The 1964 Bell System survey was comprised of two separate surveys
which were merged for analysis and presentation purposes. The basic
survey was the general loop survey which consisted of a simple random
sample of 1,100 main stations selected from the population of all main

3337

3338 THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

stations (45, 300, 000) as of January 1, 1964. However, since only 3.25
percent of all main stations are served by loops longer than 30 kilofeet,
only 35 samples would have been obtained to define the characteristics
of the longer loops. Consequently, a long loop survey consisting of a ran-
dom sample of 955 main stations served by loops longer than 30 kilofeet
was obtained. The data obtained from the long loop survey has been
used in those instances where characteristics are being expressed as a
function of length to permit better resolution of the characteristics for
the longer loops. In both of these sub-surveys, official telephones, foreign
exchange lines, dial teletypewriter exchange (TWX) lines and special
service lines were omitted as it was felt that their design would not be
representative of customer loop plant.

II. SUMMARY OF RESULTS

Analyses of data obtained in the 1964 loop surveys lead to six general
results.

(i) The average customer loop length is 10.6 kilofeet with only
10 percent of the main stations located beyond 21 kilofeet from their
serving office. The length distributions show a slight trend toward longer
loops between 1960 and 1964, with the average loop length increasing
by 300 feet.

(ii) The average 1 kHz insertion loss of Bell System loop plant is
3.8 dB and 95 percent of all main stations are served by loops having a
1 kHz loss of less than 8 dB. At 3 kHz, the average loss is 7.8 dB and
95 percent of the main stations have less than 17 dB insertion loss.
(in) The average noise balance of party-line loops is 56 dB, while
the balance for individual line loops is 69 dB. Only 5 percent of the
individual line loops have a noise balance of less than 50 dB while nearly
20 percent of party-line customers are served by loops with less than
50 dB of balance. The substantially lower balance for party lines is
largely due to the inherent circuit unbalancing effect caused by the use
of grounded ringers for party-line service.

(iv) The average metallic circuit noise (C-message weighted) at a
customer's station set is approximately 5.5 dBrnc including the noise
contribution of the central office wiring as well the noise contribution
of the outside plant facilities. Only 8 percent of the individual lines
have noise in excess of the Bell System objective of 20 dBrnc. However,
18 percent of the party-line customers have circuits which have noise in
excess of 20 dBrnc because of the generally poorer circuit balance of
party-line circuits.

CUSTOMER LOOP PLANT 3339

(v) Comparison of measured and calculated transmission char-
acteristics of Bell System loop plant has demonstrated that the outside
plant cable records are sufficiently accurate to permit characterizing
the loop plant transmission performance by theoretical calculations
based on the physical composition of the loops as described in the outside
plant records.

(vi) Main stations served by loops in excess of 30 kilofeet in length
were found to be exponentially distributed as a function of working
length, with the population of main stations reduced by 50 percent with
every 11-kilofoot increase in loop length (see Fig. 6). It is estimated
that 1.5 million Bell System customers (3.25 percent of all customers) are
presently served by loops in excess of 30 kilofeet in length. Due to the
party-line character of longer loops, the 3.25 percent of all Bell System
main stations included in the long loop segment of plant used only
1.7 percent of the working Bell System exchange lines.

III. DESIGN OF THE SURVEY

The first steps in the survey were to define the population to be
sampled and to obtain a complete list of the sampling units. In these
two surveys (that is general loop and long loop), main stations were
selected as the sampling units and all Bell System main stations as of
January 1, 1964, were taken as the population to be sampled. A simple
random sample was chosen as the sampling plan.

The sample size of the general loop survey was selected to provide
data of equal precision to that obtained in the 1960 survey. The design
parameter chosen was the average distance to the sampled main sta-
tions, and the precision was measured in terms of the width of the con-
fidence interval bounding this average distance. The desired confidence
interval (at 90 percent confidence level) of ±450 feet on the average
cable distance to the sampled main stations dictated a sample size of
1,100 randomly selected main stations. The actual confidence interval
obtained was ±476 feet.

In the long loop survey, lack of previous knowledge concerning the
composition of long loops made it difficult to accurately determine
the minimum sample size which would provide sufficient precision. The
design parameter selected for the long loop survey was the average noise
metallic (C-message weighting) measured at the telephone sets of the
sampled main stations to a dialed-up termination. The precision aimed
for was a ±1.0 dB confidence interval (at 90 percent confidence level).
A sample size of 955 main stations was collected, and the confidence
interval obtained was ±0.73 dB.

3340

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

The two surveys had satisfactorily wide geographical dispersion, with
every associated company (except Canada) contributing to the survey.
Reference to Fig. 1 will illustrate that the large companies and the
metropolitan areas contributed heavily to the general loop survey and
the rural areas contributed heavily to the long loop survey.

IV. LOOP SURVEY RESULTS — PHYSICAL COMPOSITION

Data obtained in the loop survey included detailed loop schematics
indicating the loop composition of each of the loops sampled in the
survey. All distributions of physical quantities discussed herein were
derived by analysis of these loop schematics. Since similar data were
obtained in the 1960 loop survey, comparison of the physical distribu-
tions obtained in the two surveys has also been made.

Table I gives a summary of the statistics for the principal physical
properties of loop plant. Data are included for both the 1960 and 1964
surveys and significance levels for differences of mean values are pre-
sented when meaningful. Cumulative distributions of these factors are
shown in Figs. 2 through 5. The distribution of working bridged tap
is not given since 82 percent of the sampled main stations were served
by loops having zero working bridged tap and consequently the dis-
tribution is not particularly enlightening.

As indicated in Table I, the estimated average route distance from
serving central office to main station in the Bell System is 10.6 kilofeet
with 90 percent confidence that the true mean value lies within ±476
feet of this estimate. Note that although the estimated mean working
length in 1964 is over 300 feet longer than that estimated in 1960, it is
not statistically possible to claim that the observed increase is indeed

Table I — 1964 Customer Loop Survey Summary
of Main Station Characteristics

Main Station
Quantity

Mean (ft)

90% Confidence
Limits on Mean (± ft)

Sign. Level for

1960

1964

1960

1964

in Percent

Working length
Total bridged tap
Working bridged tap
Airline distance
Working length/
airline distance

10,288

2,619

381

7,604

1.45

10,613

2,478

228

7,758

1.50

450
169
107
353

0.02

476

172

74

386

0.03

*
*

95

*

98

Drop wire excluded except when individual lengths exceed 400 feet.
* Levels of significance less than 80 percent indicated by asterisk.

CUSTOMER LOOP PLANT 3341

an increase. Reference to the cumulative distribution of working length
depicted in Fig. 3 will, however, show that shifts in the distribution have
occurred since 1960. Note that the percentage of longer loops increased
from 1960 to 1964.

Analysis of the long loop survey data has shown that the Bell System
main stations served by loops in excess of 30 kilofeet are exponentially
distributed as a function of working length as depicted in Fig. 6, with
the main station population (iiminishing by 50 percent with each 11
kilofeet increase in loop length. Survey analysis indicates that about
1.5 million or 3.25 percent (with 90 percent confidence interval of
±0.2 percent) of all Bell System main stations were located 30 kilofeet
or more from their serving central offices as of the end of 1964. Due to
the party-line character of the longer loops, the 3.25 percent of all Bell
System main stations included in the long loop segment of plant used
only 1.7 percent of the 39,300,000 Bell System lines working in 1964.

Analysis of the survey data has also provided valuable insight into
the type-of -service distribution of Bell System customers and the physi-
cal composition of the plant provided to meet this distribution as shown
in Figs. 7 to 10. The type-of-service distribution was derived as a
function of length to the sampled main station and took advantage of
the pooling of data from the two surveys. To evaluate the physical
composition (type of facility, gauge, and pair size) of the loop plant as
a function of distance, the sampled loops from the general loop survey
were inspected at intervals of 1,000 feet starting at the central office.
Both the general loop survey and the long loop survey were similarly
inspected to define these distributions beyond 30 kilofeet.

The extent of party-line development as a function of loop length
is shown in Fig. 7. Note the rapid increase in eight-party development
for loop lengths greater than 30 kilofeet. Examination of the pair size
distribution as a function of distance from the central office (Fig. 8)
shows rapidly decreasing pair size with distance (at the 50-kilofoot point
50 percent of the sampled loops are contained in cables with fewer than
16 pairs). Similarly, the distribution of gauge shown in Fig. 9 illustrates
a rapid transition to coarse gauge with increasing distance from the cen-
tral office. For example, at 30 kilofeet 60 percent of the sampled loops
are composed of gauges coarser than 22 gauge. Note also (Fig. 10) that
the longer loops are primarily developed with aerial facilities. For
example, 78 percent of all plant is aerial at the 30-kilofoot point from the
central office. For the longer loops where small pair sizes are used, the
pole line costs become a significant portion of the total loop costs and
this factor is one of the reasons for the present trend towards the use

3342 THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

of buried plant. Since the sampled loops were randomly selected from
all existing plant, buried plant is not as prominent as it would be in a
sample of new construction. Note, however, that beyond 30 kilofeet,
buried facilities in 1964 accounted for approximately 20 percent of the
loops.

V. 1964 LOOP SURVEY RESULTS — TRANSMISSION PERFORMANCE

Data obtained in the 1964 loop survey have provided considerably
more comprehensive knowledge of the transmission performance of
customer loop plant than heretofore available. In the 1960 loop survey
all transmission performance data were developed by deriving equivalent
"T" networks from the information supplied on the loop sketches and
analyzing these networks for transmission performance at nine fre-
quencies in the voice band. Similar analysis has been performed for each
of the sampled loops in the 1964 loop surveys, and in addition trans-
mission measurements were made. The measurements covered noise,
crosstalk, insertion loss at 1, 2 and 3 kHz, and dc resistance. The com-
bination of these two sets of transmission performance data (one cal-
culated and one measured) permits three types of analysis:

(i) changes in transmission performance since 1960 by comparison
of calculated 1960 data with calculated 1964 data,

(m) comparison of measured versus calculated data for the 1964
survey, and

(Hi) provision of heretofore unavailable data on the noise and cross-
talk performance of customer loop plant.

Since measured insertion loss data was not obtained in the 1960
survey, comparison of 1960 and 1964 data must be based on calculated
values. Figure 11 depicts the 1 kHz calculated distributions for both
surveys. It can be seen that insertion loss performance has remained
virtually unchanged since 1960.

For those transmission characteristics where measured data are
available in addition to the analytically derived data, minor differences
in performance are exhibited by the two distributions of data (Fig. 12).
In this regard it is important to realize that the measured data should
provide a more accurate estimate of performance. There are several
reasons for greater confidence in the measured data. First, possible in-
accuracies in cable records or errors in transferring data from the records
to the loop sketches can introduce errors in the calculated data. Second,
errors in construction, such as omission or improper connection of load-
ing coils, cannot be detected from the records. Third, use of calculated

CUSTOMER LOOP PLANT 3343

data assumes that all cables exhibit nominal characteristics and con-
sequently do not reflect manufacturing tolerances and environmental
factors.

The cumulative distributions of insertion loss at 1, 2, and 3 kHz for
customer loop plant as derived from both measured and calculated
data are presented in Fig. 12. These insertion loss measurements and
calculations were made with a 900 ohm source and load as depicted in
Fig. 13. Measured loss was found to be consistently higher than calcu-
lated loss across the entire voice frequency band. The absolute dif-
ferences between measured and calculated losses are small however, as
indicated by the differences in mean losses. For example, at 1 kHz the
measured loss was 3.8 dB and the calculated loss was 3.5 dB. This com-
parison of measured and calculated insertion losses demonstrates the
feasibility of characterizing the loop plant transmission performance by
theoretical calculations based on the physical composition of the loops
as described by outside plant records. Still referring to Fig. 12, note
that approximately 95 percent of all Bell System main stations are
served by loops having a 1 kHz insertion loss of less than 8 dB with a
mean loss of 3.8 dB. Similarly, at 3 kHz the 95 percent point is 16 dB
and the mean loss is 7.8 dB. A scatter diagram of the 1 kHz measured
insertion loss as a function of loop length is shown in Fig. 14. This dia-
gram was obtained by merging the data from both the general loop
survey and the long loop survey and indicates that the high loss loops are
not limited to the long loop category. The high losses observed on some
of the short loops generally reflect excessive bridged tap.

An insertion loss measurement of particular interest to designers
of data equipment is the slope of loss versus frequency from 1000 to
2750 Hz. Cumulative distributions of the 2750 - 1000 Hz insertion
loss (insertion loss measured with 900 ohm source and load) have been
provided for all Bell System loop plant and for those loops serving
business customers in Figs. 15 and 16 respectively.

Another important transmission characteristic is return loss, signifi-
cant from echo and singing considerations. Return loss performance was
not available from the measured data; consequently, it was cal-
culated. Table II provides 1964 loop survey return loss results for nine
frequencies and Fig. 17 presents the cumulative distributions and
histograms for the 3 kHz singing return loss and echo return loss (equal
weighting of the 500 to 2,500 Hz band). These data are all developed
on the basis of looking into the customer loop at the central office end
of the loop. The return loss is obtained by matching against a 900 ohm,
2.16 jiF termination at the central office, with the customer end of the

3344

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

Table II — Calculated Return Loss From Office
Toward Station*

1964

90% Confidence

Frequency (Hz)

Mean (dB)

Interval (± dB)

200

8.0

0.11

300

10.2

0.12

500

13.4

0.17

1,000

15.4

0.30

1,500

13.1

0.27

2,000

10.9

0.25

2,500

9.1

0.20

.'5,000

7.7

0.16

3,200

7.1

0.15

Echo

11.2

0.15

Station end of loop terminated in impedance of "off-hook" station set.

loop terminated in the impedance of the "off -hook" station set. Similar
data on return loss are presented later from the station end of the loop.

The comparison of measured versus calculated loop resistance shown
in Fig. 18 indicates that for general loop plant there is no significant
difference between measured and calculated data but calculated loop
resistance is slightly higher than measured resistance (574 ohms cal-
culated, 567 ohms measured). Measured values may have been in-
fluenced by the fact that measurements were made during the winter.
Calculated resistances were based on an average temperature of 68°F.

Theoretical calculations cannot be made of all transmission perform-
ance characteristics. Two such examples are noise and crosstalk. Since
these characteristics are dependent upon external influences (induction
from adjacent power lines, cable pair balance and the particular pair
assignment), field measurements were made on each of the sampled
loops using a Western Electric Company model 3A noise measuring set.
The noise and crosstalk measurements were made as depicted in Figs.
19 and 20.

It is convenient to analyze loop noise in terms of the two factors
which contribute to the resultant interference. The first of these is the
magnitude of open circuit longitudinal voltage induced from power
lines and the second is the circuit balance of the cable pairs and central
office equipment. The cumulative distribution of the open circuit longi-
tudinal voltage for general loop plant is shown in Fig. 21 for 3 kHz flat
weighting. This voltage is induced in a longitudinal mode, and conse-
quently only that portion of it which is converted to the metallic circuit

CUSTOMER LOOP PLANT 3345

will create an interference problem. The circuit balance reflects the
extent to which the longitudinal voltage is converted to metallic voltage
and is, therefore, a measure of the susceptibility of the telephone plant
to inductive interference such as power-line hum.*

As seen in Fig. 22 party lines are much more susceptible to power-
line hum than individual lines because of the unbalance introduced by
the grounded ringers associated with party-line station sets. On the aver-
age, individual lines have approximately 12 dB better balance than party
lines. Part of this is a result of the shorter length distribution of in-
dividual lines which offers less opportunity for cable pair unbalances
to accumulate; but it is reasonable to expect a balance improvement of
10 dB with ringers isolated from ground.

The combination of the induced longitudinal voltage and the circuit
unbalance produces the metallic noise distribution at customers' station
sets (in off-hook state) as shown in Figs. 23 and 24. For comparison
purposes, the C-message weighted noise to ground (longitudinal noise)
is also shown on these figures. Figure 23 depicts the noise contribution
of the loop plant only, while Fig. 24 includes the noise contribution of
the central office wiring. In both cases the station end of the loop was
terminated in an off-hook 500-type station set with the transmitter
and receiver replaced by equivalent resistors. The metallic noise has been
measured with C-message weighting to reflect the relative interfering
effects of the noise on voice transmission. The important limits to con-
sider are the Bell System long-term noise performance objective of 20
dBrnc and the immediate remedial action limit of 30 dBrnc. As seen
in Fig. 24, only 8 percent of the individual lines had total metallic
noise in excess of 20 dBrnc. However, 18 percent of the party-line cus-
tomers have noise in excess of 20 dBrnc.

The near-end crosstalk coupling loss characteristics of customer loop
plant as derived from measured data from the general loop survey are
shown on Fig. 25. Along with the overall distribution of crosstalk
coupling loss is shown the distribution for nonloaded loops only (84
percent of all sampled main stations) . The nonloaded loop distribution

* Loop circuit noise balance is defined here as

9n . open circuit longitudinal voltage

metallic voltage

where both voltages are measured with C-message weighting. The validity of this
definition depends on the assumption that the longitudinal voltage induced from
adjacent power lines is the only source of metallic noise. This is generally not true
when the noise to ground measures less than 20 dBrnc, and consequently loop
balance for such loops cannot be computed from the measurements.

3346 THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 196t)

can be approximated by a normal distribution with a mean crosstalk
coupling loss of 115 dB and a standard deviation of 12 dB. A comparison
of these two curves indicates that the poorer crosstalk performance of
longer loaded loops dominates the low loss tail of the general loop survey
distribution.

The final transmission characteristic to be discussed is the loop input
impedance as calculated both at the central office and at the station set.
Figure 26 presents the plot of loop input impedance as seen at the
central office as a function of frequency (not including central office
wiring or equipment). For these calculations the station end of the
loop was terminated in an off-hook 500-type station subset with the
transmitter and receiver replaced by equivalent resistors. Curves have
been provided separately for loaded and nonloaded loops because of
the large difference in their characteristic impedance. Also shown is the
characteristic impedance of the central office matching network as a
function of frequency. The function of this network is to provide high
return loss performance across the voice frequency band by matching
as close as possible the impedance of the various loops. It is apparent
that both the nonloaded loops and loaded loops should have their
highest return losses around 1 kilohertz and that the loaded loops
should perform more poorly than nonloaded loops at the lower fre-
quencies.

Plots of mean input impedances, such as in Fig. 26, are useful for
indicating the general input impedance behavior as a function of fre-
quency. Variations that occur at each frequency, and their effects on
return loss, are best shown as scatter diagrams. Figures 27 and 28
present the loop input impedance at 1 kHz for nonloaded and loaded
loops. Superimposed on all scatter diagrams are return loss circles
referenced to the 900 ohm and 2.16 /jF matching network. Any loop
having an impedance lying within a particular circle will have a return
loss, when measured against the specified matching network impedance,
which exceeds the given return loss value. Visual examination of the
scatter pattern as it relates to the return loss circles provides a ready
means of evaluating the return loss performance of various segments of
the loop plant (assuming, of course, that the input impedances of loops
in that segment are known).

The range and shape of the input impedance scatter pattern at each
frequency are of interest because they point up the difficulty of designing
a simple matching network which, at even a single frequenc3 r , will
provide very high return losses for nearly all loops. Considering the
characteristics of the nonloaded loops shown in Figure 27 it is evident

CUSTOMER LOOP PLANT 3347

that many of the loops tend to follow a smooth curve, while the others
are scattered about this curve. The smooth curve results from the varia-
tion in loop length, while the scatter is due to the effects of bridged tap,
overgauging, and variations in types of subsets.

Perhaps of particular interest to Bell System customers are the input
impedance characteristics of Bell System loop plant as seen from the
station end of the customer loop. The input impedance of a customer
loop as measured at the station set can vary considerably based on the
type of facility connected to the loop at the central office. Various cir-
cuit connections may involve use of four-wire trunks, two-wire trunks
or intraoffice circuits. In the following analysis a 900 ohm and 2.16 nF
central office termination has been used to represent a four-wire trunk
termination, and the midsection input impedance of 22 gauge H88
loaded cable has been used to represent a two-wire trunk. For the simula-
tion of intraoffice calls, a Monte Carlo technique was used to select a
random sample of 500 pairs of loops from the 1,100 loops in the general
loop survey. A loop was randomly selected as the sample loop, and then
the input impedance (from the central office) of another randomly
selected loop was chosen for the central office termination.

The 1,100 loops of the 1964 general loop survey were segregated into
two groups (loaded and nonloaded loops) for all but the simulated
intraoffice calls because of the great differences in impedance range of
the two populations. Presentation of scatter diagrams of input im-
pedance from the station set has been limited to 1 and 3 kHz. These
return loss circles were generated assuming the use of a 500-type station
set and it was further assumed that the 500 set was operating on a cur-
rent equal to the average loop current of 45.5 mA.

Figures 29 through 32 are the input impedance scatter diagrams for
loops with a simulated two-wire trunk (22 gauge H88 loaded cable)
termination at the central office. The scatter is primarily a result of
overgauging, open wire, and bridged tap or varied end section length.
Smoothed curves of the mean input impedances of loaded and non-
loaded loops with a 22 gauge H88 cable termination are presented in
Fig. 33. Scatter diagrams for the loops with a central office termination
of 900 ohms and 2.16 nF (simulated four-wire trunk) are presented in
Figs. 34 through 37. The general input impedance behavior of these
loops as a function of frequency is indicated in Fig. 38 by the plot of the
mean input impedances at nine voiceband frequencies.

The scatter diagrams and the mean input impedance curve for the
simulated intraoffice calls are shown in Figs. 39 and 40. The effect of
connecting together two loops, one of which is terminated by a station

3348 THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

set, and the other whose input impedance is calculated at its station
set, is shown by the mean input impedance curve for simulated intra-
office connections in Fig. 41. This curve has a shape characteristic of
longer nonloaded loops. The mean input impedance curves for non-
loaded loops with simulated two- and four-wire trunk terminations at
the central office are also shown in Fig. 41. The major differences in the
characteristics of these curves are at the low frequencies where the shunt
capacitance of the cable masks the termination less than it does at high
frequencies.

VI. ACKNOWLEDGMENTS

The author wishes to acknowledge the collaboration of Mr. F. L.
Schwartz in the design and execution of this Bell System customer loop
survey and his analysis of noise and crosstalk characteristics of loop
plant. I also want to thank Mrs. A. F. Rogers for the substantial pro-
gramming assistance required in the analysis of the data obtained in
this study and Mr. D. B. Menist and Mrs. B. J. Hymanson for their
contributions on the input impedance characteristics of loop plant.

REFERENCES

1. Hinderliter, R. G., "Transmission Characteristics of Bell System Subscriber

Loop Plant," AIEE Summer General Meeting, Denver, Colorado, June
17-18, 1962, Paper 62-1198.

2. Nasell, I., "Some Transmission Characteristics of Bell System Toll Collec-

tions," BJ3.T.J., 47, No. 6 (July-August 1968), pp. 1001-1018.

CUSTOMER LOOP PLANT

3349

34-14

Fig. 1 — Geographic distribution of sampled loops for the 1964 customer loop
survey. Eleven hundred loops were sampled for the general loop survey and 955
loops were sampled for the long loop survey.

3350 THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

20

-C^"

^ — — -

I96C

//196

4

/

/

ll
II
II

ll
ll
II
ll

f\

It
ll
II
II

20 25 30

LENGTH IN KILOFEET

35

40

4 5

Fig. 2 — Working length to the main station.

1960 1064

Mean (feet) 10,288 10,613
90 percent confidence

limits on mean (feet) ±450 ±476

CUSTOMER LOOP PLANT

3351

(00

z 70

o

.?. 40

1964

•""i960

/"

/

/

6 8 10 12 14

LENGTH IN KILOFEET

Fig. 3 — Total bridged tap.

Mean (feet)
90 percent confidence
limits on mean (feet)

1960
2,610

±169

1964

2,478

±172

3352

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 196S)

60

•

^

.— •— ' •""

19

60//19
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S4

f
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if

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i

1

II
II
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II
ll
II
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5 10 15 20 25 30 35 40

LENGTH IN KILOFEET

Fig. 4 — Airline distance to main station.

1960 1964

Mean (feet) 7,604 7,758

90 percent confidence

limits on mean (feet) ±353 ±386

CUSTOMER LOOP PLANT

3353

iOO

1.4 1.6 1.8 2.0 2.2 2.4 2.6

RATIO OF WORKING LENGTH TO AIRLINE DISTANCE

Fig. 5 — Ratio of working length-airline distance to main station.

1960 1964

Mean 1.45 1.50

90 percent confidence

limits on mean

±0.02

±0.03

3354

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

I.SXIO 6

(50,000 $5

15,000

70 90 110 130 150 170 190

LENGTH TO MAIN STATION IN KILOFEET

210

Fig. 6 — Distribution of long loops (1964 long loop survey). The mean was
45,938 feet; 90 percent confidence limits on the mean was ±870 feet.

CUSTOMER LOOP PLANT

3355

16 20 24 28 32 36 40

LENGTH TO MAIN STATION IN KILOFEET

Fig. 7 — Type of service distribution versus loop length (1964 combined loop
surveys) . One-, two-, four-, and eight-party plots include residence service only ;
business includes PBX, centrex, and coin.

3356

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

E 60

<
tfl

O 50

£ 40

^3

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O / Q

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/ OS /

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Hi

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20 30 40 50 60 70 80

DISTANCE FROM CENTRAL OFFICE IN KILOFEET

90

Fig. 8 — Pair size distribution (1964 combined loop surveys— general loop and
long loop surveys).

CUSTOMER LOOP PLANT

3357

20 30 40 50 60 70 80

DISTANCE FROM CENTRAL OFFICE IN KILOFEET

Fig. 9. — Gauge distribution (1964 combined loop surveys— general loop and
long loop surveys) .

3358

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

100

60
50

30

S \OPEN WIRE

AERIAL

RURAL

-URBAN
RE

CABLE

10

UNDERGROUND }
CABLE

BURIED
CABLE

10 20 30 40 50 60 70 80 90 100

DISTANCE FROM CENTRAL OFFICE IN KILOFEET

Fig. 10 — Type of construction distribution (1964 combined loop surveys —
general loop and long loop surveys) .

CUSTOMER LOOP PLANT

3359

« 80
O

O 40

ikHz

^

/

2 KHz
/y

/

3 kHz

ft

//

//

/

f

ji

' A >

//
y

J

Y

8 10 12 14

INSERTION LOSS IN DECIBELS

Fig. 11 — Distribution of calculated insertion losses at 1, 2, and 3 kHz

nfidence
jan (dB)

1kHz

2 kHz

3 kHz

Mean (dB)
90 percent co
limit on m

1960 1964
3.4 3.5

±0.11 ±0.10

1960 1964
5.4 5.3

±0.18 ±0.16

1960 1964
7.4 7.3

±0.24 ±0.21

3360

THE BELL SYSTEM TECHNICAL JOUENAL, DECEMBER 1969

6 8 10 12 14 16

INSERTION LOSS IN DECIBELS

Fig. 12 — Distribution of measured and calculated insertion losses at 1, 2, and
3 kHz ( calculated; measured).

1 kHz

Meas- Calcu-

ured lated

2 kHz
Meas- Calcu-
ured lated

3.8

3.5

5.6

5.3

Mean (dB)

90 percent confidence

limit on mean (dB) ±0.12 ±0.10 ±0.17 ±0.16

3 kHz

Meas- Calcu-

ured lated

7.8 7.3

±0.22 ±0.21

CENTRAL OFFICE

OUTSIDE PLANT

Tf

CUSTOMER
LOOP TO BE
MEASURED

ON -HOOK
STATION SET

3A NOISE
MEAS. SET

Fig. 13 — Insertion loss measurement technique. The oscillator is set for 600
ohm output termination; the 3 A noise measurement set is equipped with 900
ohm termination.

CUSTOMER LOOP PLANT

3361

lb

12

Ul

_i

UJ
CD

"*■•."■ '

•.-'• ' '

UJ

Q

Z

*. .'. .

7"

.MEAN
• LOSS

C^

g 8
O

'.t ' '

.', . . .

• ':.•

t .' X ". J

z
o

fe 6

UJ

m
Z

'.'"•:.'■ ■

•■•Z^^"""

ti!fc. a l"

v,vy.*; :

.V-.

w«

1 .

• :•'•■'■

2f ' "

::..-..

2

i .' /Vii'"~

"JR*

p 7 - •

30 40 50 60 70 80

LENGTH TO MAIN STATION IN KILOFEET

Fig. 14 — Measured 1 kHz insertion loss scatter diagram (1964 combined loop
surveys — general loop and long loop surveys).

3362

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 19(59

100

O 50

c

J/"^

A

/

/

/,

/

/

11

/

>

1

1

I 2 3 4 5 6 7

INSERTION LOSS SLOPE IN DECIBELS

Fig. 15 — Distribution of insertion loss slope between 2750 and 1000 Hz for
residential plus business loops.

CUSTOMER LOOP PLANT

3363

o

H 40

z

^•^i

/

I

/

/

/

1

2 3 4 5 6 7

INSERTION LOSS SLOPE IN DECIBELS

Fig. 16 — Distribution of insertion loss slope between 2750 and 1000 Hz for
business loops only.

3364

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

(/i 70

Z

o

30

20

10

7\ \

ECHC

RET

JRN I

-OSS,

At

<HZ F

*ETUF

N LC

SS

/

/

/

/

/

/

/

/

/

s

/

J

10 12 14 16 18 20
RETURN LOSS IN DECIBELS

22 24 26 28 30

Fig. 17 — Distribution of 3 kHz and echo return losses at central office. Echo
return loss distribution assumes flat weighting of 500-2500 Hz band.

Mean (dB)
90 percent confidence
limits on mean (dB)

Echo
7.7

±0.16

3 kHz
11.2

±0.15

CUSTOMER LOOP PLANT

3365

100

t- 60

4-
<

5 50

u
o

H

.?. 40

30

20

MEASUR

ED— J/

•-CALCULATED

w

/

400 600 800 1000 1200 1400 (600

RESISTANCE TO MAIN STATION IN OHMS

1800 2000

Fig. 18 — Measured and calculated distribution of resistance to main station.

Measured Calculated

Mean (ohms)

90 percent confidence

limits on mean

567
±15.8

574
±17.0

3366

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

OUTSIDE "OFF-HOOK" 500-TYPE

PLANT SET WITH TRANSMITTER

AND RECEIVER REPLACED
BY RESISTORS

7T

CUSTOMER

LOOP TO BE

MEASURED

3A
SET

900 n

TERMINATION

900 a

TERMINATION

CUSTOMER
LOOP TO BE
MEASURED

3A
SET

X

Fig. 19 — Noise measurement technique for (a) noise metallic — loop only and
(b) noise longitudinal — loop only.

CUSTOMER LOOP PLANT

3367

OUTSIDE "OFF-HOOK" 500 -TYPE

PLANT SET WITH TRANSMITTER

AND RECEIVER REPLACED
BY RESISTORS

CUSTOMER

LOOP TO BE

MEASURED

3A NOISE 900 O.

MEAS. SET TERMINATION

"OFF- HOOK"
STATION SET

Tt

DISTURBING
LOOP

Fig. 20 — Crosstalk measurement technique. Disturbing loop — randomly se-
lected pair in the same 100-pair group as the sample loop on which the measure-
ments are being made. The oscillator on disturbing loop is set for 600-ohm
output termination.

3368

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

z
o

i-
?, 40

o

34 42 50 58 66 74 8 2 90

0.1 1 5 10 25 50

NOISE TO GROUND (3A NOISE MEASURING SET READING)

Fig. 21 — Noise to ground at main station for 3 kHz flat weighting. The mean
was 49.2 dBrn; the 90 percent confidence limits on the mean was ±0.56 dBrn.

CUSTOMER LOOP PLANT

3369

20

44 50 56 62 68

LOOP BALANCE IN DECIBELS

92

Fig. 22 — Loop circuit noise balance. These distributions are based on the 476
loops where measurements permitted an accurate estimate of loop balance to be
made. These are for loops having noise to ground greater than 20 dBrnc.

Mean (dB)
90 percent confidence
limits on mean (dB)

Party
56.1

±1.55

Individual
6877

±0.87

3370

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

100

if) 80

Z

5

z
<

INDIVIC
LINE

UAL
S ~^y

j£ NOISE TO GROUND
f^~~~ (LONGITUDINAL)

/

' \

LL

°40

\-
z

LLI
O

' \

/

PARTY
LINES

ALL

LINES

/

Z

a. 20

-12

6 12 18 24 30

NOISE METALLIC IN dBmC

42

48

Fig. 23 — Noise metallic at main station with C-message weighting for loop
plant only.

Noise Metallic

Noise to —

Ground All Individual Party

Mean (dBrnc) 19.1 -1.1 -3.1 ' 6.7

90 percent confidence

limits on mean (Bdrnc) ±0.5 ±0.5 ±0.5 ±1.3

CUSTOMER LOOP PLANT

3371

-12

-6

6 12 18 24 30

NOISE METALLIC IN dBlTIC

36

42

48

Fig. 24 — Noise metallic at main station with C-message weighting for loop
plant plus central office.

Noise Metallic

Mean (dBrnc)

90 percent confidence

limits on mean (dBrnc)

Noise to

Ground

19.1
±0.5

All
5.6

±0.5

Individual
4.3

±0.6

Party
10.6

±1.2

3372

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

(S 99.99

<;

SN

^,-ALL LOOPS

NO
LOC

NLOAC

ips or

ED

JLY

-"" 'v

110 105 100 95 90 85 80 75 70

NEAR END CROSS TALK COUPLING LOSS IN DECIBELS

Fig. 25 — 1 kHz near end crosstalk coupling loss at central office. Central of-
fice terminated in 900 ohms; customer end terminated in receiver off hook sta-
tion set.

-600

400

600

1000 1200 1400 1600

RESISTANCE IN OHMS

1800 2000 2200

Fig. 26 — Loop impedance at central office.

CUSTOMER LOOP PLANT

3373

200

/

/

/

/./

/

\

/

/

%/"

'%?'■•;

• • ;•:

\

K ". .-•

I • *

• /*.'

\

"_.■

"" t

.'■ <

in

5

\

\

■ /

z

V

\I2

dB •

.* •!•••■

z

U -500
UI

jodB

• ,\* &

/".

y

\ 8 dB .• v

\60

B "^

...■*.•'%

:"•&

•^

■}■ ' '•

'-'

i4dB

■*•' *

; ; ! \;

' * » s

. • ■

•'

-1000

200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
RESISTANCE IN OHMS

Fig, 27 — Nonloaded loop input impedances at 1 kHz measured from the cen-
tral office. Return loss circles based on 900 ohm + 2 juF network.

3374 THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER !969

o

z

uj o

o

z

o

< -200

UJ

cr

-400

AdB

/edB

/adB

/

odB^

//

y\zt

JB *

//

7/

.

\\

t

v.

•;

/

\

*-■*"■■— ii

-"• -

<S\ .

/

/

\

V

■ •

/

200 400

600 800 1000 1200 1400 1600 1800

RESISTANCE IN OHMS

Fig. 28 — Loaded loop input impedances at 1 kHz as measured from the

central office. Return loss circles based on 900 ohms + 2 jiF network.

CUSTOMER LOOP PLANT

3375

100

w

\ \

\

V

\\

-100

W

N

\

.

-200

\

\\

\

s

7^

-300

— ^

\\

•

•'"■■•V

m

. ••%

I

o

?-500

:N

t •. ■

I*??*

L "•"

%;.:■•:

&;:

^

• .-' :

N ..

5

<J

<

o

•^

fflxA

'$£'.';

• . s

•v •

a

■>M

fev:;

>** •••

3

v2

-1100

\1

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300
RESISTANCE IN OHMS

Fig. 29 — Nonloaded loop input impedances at 1 kHz measured from station
set with a simulated two-wire trunk termination at the central office. Return loss
circles based on 500-type subset impedance.

3376

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1909

-100

-200

-30C

-400

uj-500
U

z
<

a:

600

-700

-800

-900

-1000
-1100

\

\

y

\

\

\\

\

\

• \

■\\

i A.

■ \

V

\

M

&-

•\

\

V

S?" A

\

nJ

s7

?w..

-

'a

\5

\ 1
MS

v ' .4,

\ 3

\i

\2

^". J

y'- .••:

•

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

RESISTANCE IN OHMS

Fig. 30 — Nonloaded loop input impedances at 3 kHz measured from station
set with a simulated two-wire trunk termination at the central office. Return
loss circles based on 500-type subset impedance.

CUSTOMER LOOP PLANT

3377

200
100

-200

O -300

w -400

-800

-900

-1000

y\

\\

ss

/

\\

7^"

\\

X

' ~

•1 — ^

\/'

1

V

V

^v

.' . t

\' "

"is*"""

-• — :

5

\

'

\

9^3

^

\

v2

K

\

00 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
RESISTANCE IN OHMS

Fig. 31 — Loaded loop input impedances at 1 kHz measured from station set
with a simulated two-wire trunk termination at the central office. Return loss
circles based on 500-type subset impedance.

3378

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

\\

\

v

V

V

\\

\

\

\

\\

• \

\

>,

\ '. \

\

\

12 -/nn

v

V

i
O

Z -500

■•■\

* •

V'\

\

S^7

\ ^

.

UJ

■k

5 s

E

3

\

\

2\

\

-1100

\

\

\

">•

O 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

RESISTANCE IN OHMS

Fig. 32 — Loaded loop input impedance at 3 kHz measured from station set
with a simulated two-wire trunk termination at the central office. Return loss
circles based on 500-type subset impedance.

2 -500

? -600

320

DHZ

2

(A

500 H 7
"200C

Lfl.

H7,

qIOOO

HZ

320

HZd

ap<o

N

L LOADED
\ 1

>q500 H

Z,

100

HZ
NO

•JLOAD

ED*

500 V

\

^S

20C

HZ*>

\

-1000

100 200 300 4CO 500 600 700 800 900 1000 1100 1200 1300

RESISTANCE IN OHMS

Fig. 33 — Mean value of loop input impedance from station set with simu-
lated two-wire trunk termination at the central office.

CUSTOMER LOOP PLANT

3379

-200

uj -500

U -600

-900

w

\

~v

\

\\ v

\

\ v

\\

*""•..""

\\

7

\

\ •

X

* . k ;

.»

^- ,]

^8

v.-.

;*S-

" "****•■<

• 5

^

ftsl

*■■>•)•■

*•!■ * *- * *

*'••*

-.^...s

s^a

V

2\

\

\

N

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300
RESISTANCE IN OHMS

Fig. 34 — Nonloaded loop input impedances at 1 kHz measured from station
set with simulated four-wire trunk termination at the central office. Return loss
circles based on 500- type subset impedance.

3380

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

200

uj-400

u

z

g-500
<

in

-600

-1000

\V

, \

\

\\

\ '

V

\

\

\

{ -'v

■\
r

\

V

i%

k

\

■ V.-i

: \
fe."

\

\

■>\1 *n

\

K •

. .•;

<•

\ ^

" ' •""'•£■ V

••i-S:'

N s

.

7

v^S

S*

\3

\

\

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

RESISTANCE IN OHMS

Fig. 35 — Nonloaded loop input impedance at 3 kHz measured from station
set with simulated four-wire trunk termination at the central office. Return loss
circles based on 500-type subset impedance.

CUSTOMER LOOP PLANT

3381

200

\

100

A

V

5

\\

7,

o 300

\\

2

uj -400

2
<

\

S- *

G soo
<

V

Y

•\.

*"*"■•

5

a.

\

• ;S

• *i<L

-700

,3

\1

2\

-tooo

\

^^-

RESISTANCE IN OHMS

Fig. 36 — Loaded loop input impedances at 1 kHz measured from station set
with simulated four-wire trunk termination at the central office. Return loss
circles based on 500-type subset impedance.

3382

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

-200

-300

5
I
O

? -500

< -600

1-

S

8 -700

\\

\ \

\

\

\

Y

s s

\

\;'\

.'•A

W

\

• *

^

\

v ..

7

:v\

_

\

^5

\

\ •

\

\

\

\3

\

V

"N

-1100

tOO 200 300 400 500 600 700 800 900 1000 1100 1200 1300

RESISTANCE IN OHMS

Fig. 37 — Loaded loop input impedances at 3 kHz measured from station set
with simulated four-wire trunk termination at the central office. Return loss cir-
cles based on 500-type subset impedance.

-300

100 2O0 300 400 600 600 700 800 900 1000 1100 1200 1300 1400
RESISTANCE IN OHMS

Fig. 38 — Mean value of loop input impedance from station set with simulated
four-wire trunk termination at the central office.

CUSTOMER LOOP PLANT

3383

100

-200

-300

If)

O "400

A

\\

\

•

\Y

N

\

••

.

y

\

\\

7^

\

X-;-

&

• • \

5

—

\

»'•" " . v

•■■

' '-N.'

•<X

\

\

S,

\

\

L3

\

\

^

-700

-800

-900

-1000

-1100

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400

RESISTANCE IN OHMS

Fig. 39 — Input impedance of all loops at 1 kHz measured from station set
with a simulated intraoffice circuit termination at the central office. Return loss
circles based on 500-type subset impedance.

3384

THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1969

200

-100

-200
| -300

! -400

I

! -500

I -600
I

-700
-800
-900
-1000

\Y

y\

V

\\

y :

v

\\

\

\.

\'A

v

*

%

'$

pj

•>• ■•?■.'

s

';>

'.^'

7

\.

\

.^5

\

V

\

\

\a

100 200

300 400 500 600 700 800 900 1000 1100 1200 1300
RESISTANCE IN OHMS

Fig. 40 — Input impedance of all loops at 3 kHz measured from station set
with a simulated intraoffice circuit termination at the central office. Return loss
circles based on 500-type subset impedance.

CUSTOMER LOOP PLANT

3385

-300

O -500

200

700 800 900 1000

RESISTANCE IN OHMS

Fig. 41 — Mean input impedance of nonloaded loops measured from station
set with: Curve A — simulated four-wire trunk 900 ohms -f 2 fiF central office
termination; Curve B — simulated two-wire trunk 22-gauge H-88 cable central
office termination; and Curve C — simulated intraoffice calls condition.